Non-linearity in length measurement using heterodyne laser Michelson interferometry

Heterodyne laser Michelson interferometry is widely used in precision length measurement but the accuracy is often limited by the unknown size of the periodic non-linearity due to imperfect separation of the two optical frequencies. A method for measuring this non-linearity with an accuracy of about 0.15 nm using pressure scanning of the interferometer is described and results are given for three configurations of a commercial measuring system.